1 search hit

Dielectron mass spectra are examined for various nuclear reactions recently measured by the DLS collaboration. A detailed description is given of all dilepton channels included in the transport model UrQMD 1.0, i.e. Dalitz decays of &#960;, &#951;, &#969;, &#942; mesons and of the (1232) resonance, direct decays of vector mesons and pn bremsstrahlung. The microscopic calculations reproduce data for light systems fairly well, but tend to underestimate the data in pp at high energies and in pd at low energies. These conventional sources, however, cannot explain the recently reported enhancement for nucleus-nucleus collisions in the mass region 0.15GeV &#8804; Me+e- &#8804; 0.6GeV. Chiral scaling and &#969; meson broadening in the medium are investigated as a source of this mass excess. They also cannot explain the recent DLS data.